Modern data center deployments frequently extend layer 2 subnets across multiple sites. This creates a large layer 2 (L2) domain that spans multiple sites. The extended subnets are reachable from the internet protocol (IP) network and have a default gateway. There may be many potential default gateways present on the subnet.
Since the host stack points to one default gateway IP address, a First Hop Resiliency Protocol (FHRP) is used to distribute the load of egress traffic over the different available physical gateways while allowing the different physical gateways to share a common logical IP address. The FHRPs normally rely on the manipulation of address resolution protocol (ARP) responses to the hosts in order to balance the load across different gateways that would have different media access control (MAC) addresses yet the same IP address. An ARP is a network layer protocol used to convert an IP address into a physical address, such as an Ethernet address. When the Layer 2 domain in which the FHRP exists is geographically distributed, the egress physical gateway can be selected to avoid traversing the LAN extension unnecessarily.